THE DISTRIBUTIVE CHARACTERS AND ENVIRONMENTAL SIGNEFICANCE OF STRONTIUM IN CARBONATE ROCKS OF DEVONEAN IN SOUTHERN GUNAN

In Devonian period, southern Hunan was situated in the east margin of Hunan, Guangdong, Guangxi neritic shelf basin.During the middle__late Devonian epoch, the carbonate sediments were dominant in this area, which may be divided into two facies area: the platform and the platform__basin, and six such facies types as the restricted platform; open platform; reef; shelf, platform basin with slope margin and platform __basin. 250 samples of fresh carbonate rocks for this study were collected from seven sections of various localities and facies types.The contents of elements in carbonate rocks are detected by ICP__AES (model JA__1160). The strontium is one of the important trace elemint in carbonate rocks.The average Sr contents and Sr / Ca × 1000 ratio in different sedimentary facies are vary widely.The tendency of this varition is closely related to sedimentary environments, i.e.the Sr/ Ca× 1000 ratio tends to increase gradually form restricted platform to open platform to shelf and to platform__basin; and from shallow to bathyal, which is fromO.27 to 0.88 to 0.95__1.18 to 2.37.In the lagoon of restricted platform, the Sr/ Ca×1000 ratio of primary dolomites is higher than that of diagenetic dolomites. Several authers have noted that many ancent carbonate rodks have a wide range and a bimodal dstrivution of Sr and given various explanation for it. We agree upon the idea suggested by J.Veizer et al. (1973, 1974), who hold that in the different sedimentary environments the primary carbonate minerals are different, and the Sr content of these minerals is also various. During the diagenetic process, the content decrease with the neomorphism from instable aragonite aragonite and high-Mg calcite into stable low-Mg calcite, but the Sr content in low- Mg calcite is not to change due to it is stable. Thus a bimodal strontium distribution of carbonate rodks in various fades is caused, and may be a indicator for environments.